The present invention relates to electrical power supply systems comprising electrochemical storage cells and a charger made up by apparatus for producing electricity from a renewable energy source, such as photovoltaic cell panels, wind turbines or hybrid apparatus in which these two sources are associated. The invention relates more particularly to the control of charging of electrochemical cells which are supplied by apparatus for producing electricity from a renewable energy source.
Typically, a battery consists of a plurality of electrochemical cells also called secondary cells or elements, connected together in series and/or parallel. Each electrochemical cell discharges while supplying electrical power to a given application. Each electrochemical cell can be charged by a charger which supplies electrical energy to its terminals to increase the amount of electrical energy stored therein. For certain applications, notably outdoor applications not connected to an electrical supply network, the charger can be a panel of photovoltaic cells, one or more wind turbines, or hybrid apparatus.
In a battery, internal resistance of an electrochemical cell can vary from one cell to another. As a consequence, charging current can vary from one branch to the other of cells connected in parallel. Charging is consequently not uniform over all the cells of the battery. This problem of balancing charge between parallel cells in a battery is well-known.
European patent application EP-A-1,848,087 discloses an apparatus and method for balancing charging current between several electrochemical cells connected in parallel. This document proposes the use of a charge controlling unit on each parallel branch of electrochemical cells. Each control unit is adapted to measure the current entering the branch and to integrate this current over a period of time. When the integrated current exceeds a set value, charging current for this branch is interrupted.
The apparatus described in EP-A-1,848,087 is not directly applicable to the case where the charger is made up by apparatus for producing electricity from a renewable energy source. For instance, to take the case of a charger made up by a photovoltaic solar panel, delivered current varies considerably as a function of solar illumination of the panel and the source can be limited in energy. It is consequently necessary to ensure good optimization of charging current distribution. This is also the case with a charger constituted by a wind turbine or hybrid apparatus.
The apparatus disclosed in EP-A-1,848,087 solely sets out to balance the charge between the various parallel branches of the battery in order to avoid any overcharging of certain cells. But this document does not attempt to optimize the energy supplied by the charger in order to best distribute it over the various branches. The apparatus described in that document does not take into consideration the state of charge (SOC) of each cell.
Now, the problem of how to optimize the sharing out of energy supplied by a charger is real when the charger is not connected to an electrical network but rather to a source of limited power, which can be the case when the charger is made up by photovoltaic solar panels and/or wind turbines. Power available from such a charger is limited, in quantity and in time.
The graph in FIG. 1 illustrates this problem of sharing out of charging current between parallel connected cells. To facilitate understanding, the charger considered is a photovoltaic solar panel. Nevertheless, the discussion below also applies to the case where the charger is a wind turbine or hybrid apparatus. The graph in FIG. 1 shows how temperature and current is varying over time in three branches of cells connected in parallel, but it will be understood that the problem of how to share out charging current appears as soon as two cells are connected in parallel, the problem being accentuated by the number of branches in parallel.
The curves in the lower half of the graph show how current is changing over a given period (for instance one day) and the curves in the top half of the graph show how temperature in the cells is varying over the same period of time. The curve Ipv represents current supplied by the photovoltaic solar panel (the charger) and the curves I1, I2, and I3 respectively show the currents passing in each one of the three branches of electrochemical cells connected in parallel. The curves T° 1, T° 2 and T° 3 respectively show temperature in the electrochemical cells of the three branches.
The graph in FIG. 1 does indeed show that the charging current Ipv does vary considerably depending on the degree of solar illumination. The graph in FIG. 1 also shows two phases during charging:                Phase 1: the current Ipv supplied by the photovoltaic solar panel is distributed between the three branches of electrochemical cells. The difference between the currents passing in each branch depends on the internal resistance of the cells of each branch. In FIG. 1, the first branch is taking more current than the other branches (I1>I2 and I1>I2); the cells of the first branch consequently get charged more rapidly and temperature curve T° 1 indicates that this first branch of electrochemical cells is heating up as the end of charging is approached. The cells of the two other branches remain at ambient temperature and have been charged much less than the cells of the first branch.        Phase 2: the current Ipv supplied by the solar panel is diminishing (at the end of the day or cloudy conditions). The curve for current I1 indicates that the first branch is grabbing all the current even though the cells of this branch are sufficiently charged. Temperature curve T° 1 shows that this overcharging current I1 is being lost through heating up.        
When the charger is connected to an electrical network—with no limitation on power available—charging current does not diminish like that in the graph of FIG. 1. Those cells which are already charged can evacuate the overcharging current as heat through heating up while the cells of the other branches finish their charging.
FIG. 1 is a good illustration of the difficulty of, firstly, sharing out charging between several parallel branches of electrochemical cells and, secondly, of making best use of the current supplied by a photovoltaic solar panel. This consideration is equally true where the charger is a wind turbine or hybrid apparatus.
Typically, for electrical power supply systems employing a charger consisting of apparatus for producing electricity from a renewable energy source, an electronic device for controlling charging is employed. FIG. 2 is a diagrammatic illustration of such an electrical power supply system with a centralized electronic control device. Such a system is known, notably the one commercially available from Helios Technology® or Phocos®.
FIG. 2 shows a charger 2 consisting of apparatus for producing electricity from a renewable energy source, electrochemical cells 3 and a central controller 14. Central controller 14 manages charging of the electrochemical cells 3 as well as their discharge in an external application 5. In such an electrical power supply system 1, control of the electrochemical cells 3 is managed by a single centralized control 14 which distributes charging current between the various branches, using switches for instance, as a function of measurements performed on the cells. Central controller 14 needs to be dimensioned to correspond to the number of electrochemical cells 3 in the system 1. If it is desired to add or remove a cell in parallel, it is then necessary to reconfigure central controller 14. This obligation to reconfigure the system when the number of electrochemical cells changes is commercially disadvantageous with electrical power supply systems using a charger consisting of apparatus for producing electricity from a renewable energy source.
U.S. Pat. No. 5,635,816 discloses apparatus and a method for regulating battery charging current as a function of its SOC. The charger is a photovoltaic cell array. Charging current is a pulse width modulation signal. Pulse width depends on the battery SOC determined from the voltage at its terminals. This patent does not describe charging current balancing between different parallel branches of electrochemical cells. Further, the apparatus disclosed in this document is centralized apparatus which cannot be adapted in modular fashion to the number of cells connected in parallel in the battery.
U.S. Pat. No. 6,081,104 discloses apparatus and a method for regulating a battery charging current as a function of its SOC. The charger can be a photovoltaic panel or a wind turbine. The charging current is a pulse width modulated signal. Pulse width depends on the voltage at the battery terminals. This document does not describe balancing of charging current between the various parallel branches of electrochemical cells. Further, the apparatus described in this document consists of centralized apparatus which cannot be adapted in modular fashion depending on the number of cells connected in parallel in the battery.
US-A-2007/0246943 also discloses apparatus and a method for regulating charging current of a battery as a function of its SOC. The charger is a wind turbine. Charging current is a pulse width modulated signal. Pulse width depends on the SOC determined from current entering and leaving the battery. This document does not describe the balancing of charging current between different parallel branches of electrochemical cells. Further, the apparatus described in this document consists of centralized apparatus which cannot be adapted in modular fashion to the number of cells connected in parallel in the battery.
There is consequently a need for an electrical power supply system that uses a charger made up by apparatus for producing electricity from a renewable energy source which makes it possible to optimize the sharing out of charging current between the parallel branches of electrochemical cells and which can be adapted in modular fashion to the number of cells used in parallel in the system.